A solar battery is known as a clean energy source. Since a solar battery generates an electric power by sunlight, the electric power to be obtained depends on illumination and changes according to a state of the weather, unlike other batteries. The difference between the changing electric powers is detected as a difference between open-circuit voltages, for example.
With respect to this, a patent document 1 (see Japanese Laid-Open Patent Publication No. 08-297516) discloses a technique that periodically stops the operation of a DC-DC converter connected to the solar battery, and measures the open-circuit voltage, for example. A patent document 2 (see Japanese Laid-Open Patent Publication No. 2010-81711) discloses a technique that performs switching control by using a PWM (Pulse Width Modulation) signal generated based on the open-circuit voltage of the solar battery, and flows a charging current from the solar battery to a storage battery.
The DC-DC converter which changes the output voltage of the solar battery may include a soft start function that secures an electric power used for the operation of a control circuit for the DC-DC converter at start-up, and limits a rush current to an output-side smoothing circuit. At this time, the limit value of a current is set up on the basis of a lower electric power in order to have a margin of the current. The start-up time until the output voltage of the DC-DC converter reaches a given rated value becomes long since a rate of increase of a voltage is low, as compared with the case where the limit value of the current is set up on the basis of a higher electric power.
With respect to this, a patent document 3 (see Japanese Laid-Open Patent Publication No. 2007-288979) discloses a power supply device that detects an input voltage from a battery, acquires a duty and a soft start period corresponding to the input voltage from a table, and outputs a PWM signal based on the duty until the soft start period elapses. The patent document 3 discloses that the power supply device changes to a normal mode after the soft start period elapses.